1. Field of the Present Invention
The present invention relates to an inverter comprising a single type transistor, and more particularly, to an organic semiconductor inverter which can be manufactured using an organic semiconductor transistor on a plastic substrate.
2. Discussion of Related Art
Organic field effect transistors have attracted attention as a next generation device because they can be manufactured through a simple process compared to that for the conventional silicon transistors and manufactured on a plastic substrate which is flexible due to its low processing temperature. The organic field effect transistors are mainly used as a switching element of a flexible display or in a circuit such as an RF-ID circuit. When the organic field effect transistor is used as a pixel driving switch of a display, a single polar transistor (for example, a p-type transistor) may just be appropriate, but when it is used as a circuit device, a CMOS transistor which is a combination of p-type and n-type transistors may be mostly appropriate in aspects of power consumption and operating speed.
However, since organic semiconductors have not had stable characteristics and reliability with respect to an n-type device so far, inverters are generally formed to have p-type single characteristics. FIG. 3 illustrates two kinds of inverter structures which can be manufactured using only p-type devices. FIG. 3A illustrates an inverter (D-inverter) in which a load is formed of a depletion transistor and a driver is formed of an enhancement transistor. And, FIG. 3B illustrates an inverter (E-inverter) in which both a load and a driver are formed of enhancement transistors.
The D-inverter type of FIG. 3A may be more preferable than the E-inverter type in aspects of power consumption, gain and swing width, however, since an organic semiconductor cannot control a threshold voltage by doping, unlike a conventional silicon semiconductor, it is difficult to form devices which have different threshold voltage characteristics by locations on the same substrate.
Accordingly, to embody the devices having different threshold voltage characteristics by locations in the structure of FIG. 3A, complicated operations, for example, differently treating the surfaces of devices by locations, have to be conducted. Moreover, the organic semiconductors have a shortcoming in an aspect of uniformity on the same substrate, and thus there is a difficulty in manufacturing a stable inverter.
Moreover, a depletion load makes width/length (W/L) of a transistor large, and an enhancement driver makes the W/L thereof small, and thereby current may be controlled by transistor size effect. Furthermore, passivation of an organic semiconductor still remains as a problem that has to be solved for commercialization.